The present invention relates to a screening device for purifying melts of synthetic or plastic material to be fed into a high pressure plastic press. The screening device includes a housing having two opposite end walls, a throughflow boring extending transversely through the end walls and communicating with a feed channel of a high pressure plastic press, a plate or a disc supported for a shifting or a rotary movement in a gap between the end walls of the housing and being provided at the level of the throughflow boring with a series of at least four openings of which one is situated outside the housing while the remaining openings are situated within the housing, one of the openings within the housing being in alignment with the throughflow boring and the other two openings within the housing communicating with evacuation means, and either the throughflow boring or each of the openings in the series being provided with a screening disc.
A screening of the above described kind is known from the German Patent DE-PS No. 34 43 654. The objective of the prior art screening device of this kind is to guarantee the continuity of flow of the melted plastic material when a new opening of the movable plate or disc enters the area of the throughflow boring.
For this purpose, the screening device according to the German Patent DE-PS No. 34 43 654 is provided with the evacuating means which always evacuates each opening in the housing gap before its entry into the area of the throughflow boring.
However, in the construction of the device according to DE-PS No. 34 43 654 the new opening during the movement or rotation of the plate or disc in the housing gap enters the region of the throughflow boring without any transition. That means that the current of plastic melt passing through the boring momentarily develops a pressure gradient due to the fact that upon entry of a new opening in the throughflow boring the empty new opening must be first completely filled up with the melted plastic material.
The effect of the pressure gradient in many applications is not disturbing. However, in the case of extremely high requirements on the quality of processing, the aforementioned minute pressure gradient may cause noticeable disturbances or undesirable effects.